[Jillian's blog] Anatomy of an Exosolar System

A good chunk of the exoplanets that we’ve detected so far are huge, Jupiter-sized and larger. A lot of them are orbiting their stars at very short distances – it might seem strange to think that planets bigger than Jupiter are orbiting their stars closer than Mercury orbits the Sun, to the point where some of them take days or only fractions of a twenty-four hour day to complete one full orbit, but that’s what we’ve actually observed (among other really cool kinds of exoplanets). For comparison, Jupiter takes about twelve Earth years to travel around the Sun once, and these giant Jupiter exoplanets orbit in only fractions of that time. Exoplanets like these are called hot Jupiters, so named of course because while they’re “jovian” (Jupiter-like) in size, their proximity to their parent stars means that their surface temperatures are several hundred times as high as those of our outer planets. Hot Jupiters don’t start out at their sweltering homes though, and how they get there is pretty interesting.

In protoplanetary and debris disks (the millions of miles of stuff around a young star, yet to conglomerate into bigger objects like planets and asteroids), material is concentrated in rings. To maintain that ring structure (rather than have the material swirl out into thinner and thinner strands until an even distribution of matter is achieved), the rings can’t be shaped in perfectly concentric circles. Rather, each ring is tilted just a little to one side with respect to the one inside it, creating a twisting effect that causes some sections along each ring to be bunched up closer together, and some sections to be spaced out farther apart. This causes the matter in these bunched-up areas to be packed more densely than the places spread farther out. Just like how it’s easier to fit ten people into a van than it is into a sportscar, the amount of  stuff there is doesn’t change – just how it’s being packaged. It helps to take a look at the diagram to picture this, since there you can really see how the subtle tilts in each ring contribute to the swirling effect.

A diagram of the structure of spiral density waves, credit Dbenbenn and Mysid; distributed via Creative Commons License

This, incidentally, is also what gives spiral galaxies their shape – on a much bigger scale, of course!
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